build.sh: Allow readlink for path-canonicalization
build.sh: Allow readlink for path-canonicalization

Debian doesn't have realpath installed by default [1], so fall back to
the more widely available readlink [2].

[1]: http://packages.debian.org/unstable/utils/realpath
[2]: http://www.gnu.org/software/coreutils/manual/html_node/readlink-invocation.html

ostree/dockerfile,fr34k8/dockerfile,wking/dockerfile